The discussion of any work, publications, sales, or activity anywhere in this submission, including in any documents submitted with this application, shall not be taken as an admission that any such work constitutes prior art. The discussion of any activity, work, or publication herein is not an admission that such activity, work, or publication existed or was known in any particular jurisdiction.
Currently proposed systems for monitoring substances of interest, such as glucose, using small sampling and monitoring devices have a number of difficulties. For example, a microdialysis probe discussed for glucose monitoring in U.S. Pat. No. 6,091,976, Jul. 18, 2000 (M. Pfeiffer and U. Hoss) is a needle-type probe with dialysis fluid flowing in and out of the probe. The probe is inserted at a length of several millimeters underneath the skin at a shallow angle so that the probe stays in the epidermal tissue. A dialysis membrane separates the probe interior from the interstitial fluid surrounding the probe. This membrane allows diffusion of substances such as glucose from the interstitial fluid into the dialysis fluid flowing in and out of the probe. The interstitial fluid is not extracted. The dialysis fluid is then pumped to a sensor placed downstream where the glucose level of the dialysis fluid is determined. The glucose concentration of the dialysis fluid has been found to correlate with the glucose level in the interstitial fluid.
Despite the name microdialysis probe in this instance, the probe dimensions are in the millimeter range. In these proposals, the reason for using such a long probe is that the area of the dialysis membrane generally defines the amount of glucose diffusing into the dialysis fluid during a given amount of time. Generally, the detection limit of practicable glucose sensors requires a certain amount of glucose in the dialysis fluid to get reliable sensor signals. The required membrane area necessary for sufficient glucose diffusion and high sensor signals is several square millimeters and this membrane generally defines the size of the probe, which explains the large dimensions of the dialysis probes and/or needles in these discussions.
A disadvantage of using a large “micro” dialysis probe is a generally painful insertion procedure that generally requires trained personnel to implant the probe underneath the skin. Thus, present microdialysis proposals do not easily allow for painless everyday usage.
According to the World Health Organization the per capita diabetes rate in the US increased from 5.2% (world: 2.4%) in 1995 to 6.0% (2.9%) in 2000, and it is expected to reach 8.4% (4.5%) in 2030. While diabetes is the leading cause of blindness, kidney failure and non-traumatic amputation of the lower limp, other severe complications associated with hyperglycemia (high glucose levels) and hypoglycemia (low glucose levels) are nerve damage, heart disease, coma and brain damage. The traditional fingerstick test typically takes periodic samples, but this monitoring can miss periods of hyperglycemia and hypoglycemia, especially during sleep. This health risk can be avoided using a continuous glucose monitor.
Currently available continuous glucose monitoring systems include the Cygnus GlucoWatch® and the Minimed CGMS™. However, it is believed that these systems cannot provide an accurate everyday glucose level control and still require periodic fingerstick tests for sensor recalibration. The GlucoWatch® is easy to use but it relies on reverse iontophoretic interstitial fluid sampling through the skin, which is affected by fluctuating skin permeability as described in K. R. Pitzer, S. Desai, T. Dunn, S. Edelman, Y. Jayalakshmi, J. Kennedy, J. A. Tamada, R. O. Potts, Detection of Hypoglycemia with the GlucoWatch Biographer, Diabetes Care, Vol. 24, No. 5, 2001
The CGMS™ is generally not designed for daily usage; it requires trained personnel to insert the sensor under the skin, as described in E. Cheyne, D. Kerr, Making ‘sense’ of diabetes: using a continuous glucose sensor in clinical practice, Diabetes Metab Res Rev, 18 (Suppl. 1), 2002.
While frequent and long periods of hyperglycemic blood glucose levels can account for many long-term complications, hypoglycemia can cause sudden coma and brain damage. Periodic fingerstick tests often fail to detect all hypoglycemic and hyperglycemic events since glucose levels can change rapidly. In particular, nocturnal hypoglycemia often remains undetected.